Wednesday, 9 October 2013

Subpixel accuracy--00 : what is subpixel?

In the world of digital image, the smallest unit(information) you can get is pixel, it is impossible for you to access the information between pixels(sub-pixel).Fortunately, with some simple mathematical tricks, we can get over this problem.

Why sub-pixel matter?

Because sometimes we need more accuracy than the original image can provide.

ex :

Camera calibration(ex : measure homography matrix)

Stereo matching

Tracking(ex : SIFT use it to measure feature point)

How to find subpixel

graph_00(from wikepedia)

A graph is worth a thousand words. With the help of bilinear interpolation, we could measure the subpixel at ease. Our target is the value of the pixel located at (20.2, 14.5).As you see, the coordinate of the target is fractional but not integer.

Equation (1), (2) and (3) show you how to measure the value of the target. If you need more details, please check out the wiki.